The deviation of some of the incident rays for view-finding purposes can be carried out either by a semitransparent reflector intercepting the entire beam, as formed for example by a pair of cemented prisms, or by a small fully reflective mirror interposed in the path of only some of the rays. The latter solution is often preferred for economic reasons.
Conventional mirrors of the fully reflective type have lateral edges which, on being struck at a glancing angle by incident rays not directly intercepted, may give rise to objectionable reflections. To avoid this problem, it has been customary to blacken the mirror edges by coating them with a lacquer or the like.